This invention relates to a method of preparing carboxy functional silicones. More particularly this invention relates to a method for preparing carboxy functional silicones from a polyether silicone with an acid anhydride or halide.
Carboxy functional silicones are useful in many applications where water- and alcohol-soluble organopolysiloxanes are desirable. For example they are useful as emulsifying agents for the formation of aqueous emulsions of conventional organopolysiloxane fluids and in applications such as alcohol based cosmetics. Carboxy functional silicones can also function as reactants for the preparation of polysiloxane polyether copolymer surfactants which are useful additives in polyurethane foam.
Historically it has been difficult to prepare siloxanes or polyorganosiloxanes containing a carboxylic acid functionality. The synthesis of precursors to such carboxylic acid functional siloxanes such as nitriles and other acid precursors has been difficult. A previous approach that has been utilized without much success has been the hydrosilation of acrylonitrile using dichloromethylsilane or 3-butenenitrile and dichloromethylsilane.
In the art developed by Holdstock as taught in U.S. Pat. Nos. 3,182,076 and 3,629,165 carboxy functional silicones are prepared by hydrolysis and condensation of a mixture containing organotrichlorosilane, a diorganodichlorosilane, and a cyanoalkyldiorganochlorosilane. During the hydrolysis and condensation of these reactants, the various silicon bonded chlorine atoms are replaced by hydroxyl groups that intercondense to form siloxane linkages. The nitrile radical hydrolyzes to a carboxy radical. Hydrochloric acid is a by-product formed in the reaction and thus the reaction mixture is very corrosive.
Under the usual conditions of platinum catalysis of the hydrosilation reaction, the hydrosilation of nitriles to yield a cyanoalkylorganosilane is difficult. Subsequently a copper amine catalytic system was developed that facilitated the hydrosilation of nitriles. The synthesis of a carboxylic acid siloxane was achieved by the co-hydrolysis of acrylonitrile and 3-butenenitrile resulting in a siloxane having the following formula: EQU M'D.sup.x.sub.7.6 D.sup.y M'
where M' represents the portion from 3-butenenitrile and D.sup.y represents the acrylonitrile adduct.
Another synthetic route for the production of a carboxylic acid adduct consists of reacting an unsaturated acid such as 10-undecenoic acid with trimethylchlorosilane to from the silyl ester followed by a catalytic hydrosilation. A subsequent hydrolysis of the hydrosilated trimethylchlorosilylester of unsaturated acid will yield the siloxy carboxylic acid derivative, as taught in U.S. Pat. No. 4,990,643 which is herewith incorporated by reference.
A similar reaction pathway that could be utilized to provide carboxy functionalized silicones is that taught by Ryang in U.S. Pat. No. 4,381,396, herewith incorporated by reference, wherein a hydride fluid is reacted with a norbornene carboxylic acid anhydride in the presence of a platinum hydrosilation catalyst to yield silicon functionalized norbornane mono-anhydrides or di-anhydrides. Ryang teaches the use of such compounds for the synthesis of organosilicon polyimide copolymers and polydiorganosiloxane polyimide block polymers and copolymers. However, a simple hydrolytic reaction of the mono- or di-anhydride should yield a carboxylic acid functionalized norbornylsiloxane or silicone. The use of norbornyl compounds is complicated by their well-known high levels of toxicity.
These approaches have significant drawbacks. The nitriles and norbornanes are toxic materials and require extensive precautions to avoid injury to personnel conducting the preparation of the siloxy derivative. The silyl ester has a very pronounced hydrolytic instability and thus requires elaborate steps to avoid contamination by water and a consequent premature hydrolysis of the ester. These drawbacks tend to inhibit the development of large scale processes for the production of silyl or siloxy carboxylic acids. A more convenient route to the preparation of such water soluble silicon containing carboxylic acids is desirable.
Carboxylic acid silicone derivatives were prepared via the addition of alcohol substituted siloxanes with dodecenyl succinic anhydride (DDSA) to produce the corresponding carboxylic acid adducts. The DDSA silicone adducts of the alcohol substituted siloxanes were, as synthesized, not water soluble. It is hypothesized that the hydrophobic nature of the silicone backbone overrides the hydrophilic contribution of the carboxylic acid functionality. Neutralization of the DDSA silicone adduct with potassium hydroxide resulted in the formation of a very viscous water-insoluble fluid.
Carboxylic acid silicone derivatives are desirable from the standpoint that these materials may be water soluble because of the acid functionality, that they may serve as emulsifying agents for less hydrophilic silicones, and therefor useful in personal care products.